Various types of combustion may be used in an internal combustion engine. For example, spark ignition (SI) of a homogenous mixture during the expansion stroke is one example method. This method relies on a timed spark from a sparking plug in order to achieve ignition within the combustion chamber of an air and fuel mixture. Another type of combustion may be referred to as homogeneous charge compression ignition (HCCI), which occurs when the temperature of the combustion chamber exceeds the autoignition temperature for the specific fuel resulting in autoignition. HCCI can be used to provide greater fuel efficiency and reduced NOx production under some conditions.
One approach to utilizing autoignition is described in U.S. Pat. No. 6,293,246. In this approach, rather than relying on autoignition to initiate combustion, a spark assisted type of auto-ignition operation is utilized. Specifically, the approach in U.S. Pat. No. 6,293,246 relies on spark assist at all times in order to initiate autoignition of a mixture that has been raised to a temperature close to the autoignition temperature. In this example, the spark assisted combustion process requires the temperature of the gas within the combustion chamber attain a state near autoignition without achieving combustion. By firing a spark and initiating combustion in a portion of the combustion chamber, the pressure, and hence the temperature, may be increased in the entire combustion chamber. Thus, the gases which were near autoignition, are elevated to or above the autoignition temperature, thus autoignition occurs throughout the chamber. This phenomena is in contrast to spark ignition combustion in which a spark is fired thereby initiating a flame front which progresses through the combustion chamber into a mixture. In contrast, spark ignition combustion occurs in a mixture which is rich enough to sustain and propagate a flame front. Furthermore, the mixture is cool enough ahead of the flame front to resist autoignition. A sparking mechanism is then utilized to assist in initiating combustion within the chamber.
The inventors herein have recognized a disadvantage with such an approach. Specifically, conditions may exist during such spark assist operations where autoignition of the air/fuel mixture may occur prior to the initiated spark. In such situations the engine may experience degraded operation.
In one approach, the above issues may be addressed by a method of operating an internal combustion engine having a combustion chamber with a piston. The method comprises: adjusting an operating parameter of the engine so that a mixture of air and fuel in the combustion chamber approaches, but does not achieve, an autoignition temperature, and performing a spark from the spark plug so that said second mixture combusts; adjusting a timing of said spark from the spark plug; and determining whether timing of said combustion correlates to timing of said spark. In one example, based on this determining, engine operating parameters can be adjusted to ensure the timing of the spark correlates to the timing of combustion.
In this way, it is possible to achieve reliable spark assisted HCCI operation across a substantially broad range of operating conditions. Therefore improved fuel economy and reduced emissions may be obtained.
In another example, a method of operating an internal combustion engine having a combustion chamber with a piston may be used. The method comprises: adjusting an operating parameter of the engine so that a mixture of air and fuel in the combustion chamber approaches, but does not achieve, an autoignition temperature, and performing a spark from the spark plug so that said second mixture combusts; adjusting a timing of said spark from the spark plug; and adjusting an operating parameter to increase a correlation between said adjusted spark timing and timing of said combustion.
In this way, it is possible to maintain control of combustion timing during a spark-assisted autoignition operation and thereby achieve improved performance, even when various parameters may inadvertently influence combustion.